1. Field of the Invention
The present invention relates to a motor control circuit, and more particularly, to a motor control circuit for controlling a motor to stay at/exit a braking state, and an operation method thereof.
2. Description of the Prior Art
A motor has many operation states such as states for clockwise rotation, counterclockwise rotation, stationary, and braking operations. A user needs to control and drive the motor through some peripheral circuits if he/she wants to maintain or change the current operation state of the motor. FIG. 1 is a diagram showing a prior art motor and a related peripheral circuit. The peripheral circuit of motor MT includes state control circuit 110 for controlling the operation state of the motor MT, a motor driving circuit composed of transistors M1-M4, and driving current detection circuit 120. The motor driving circuit composed of transistors M1-M4 is a well-known H-bridge driving circuit, supplying a driving current to a coil within motor MT so as to rotate motor MT. State control circuit 110 issues four control signals S1-S4 to respectively turn on/off transistors of the H-bridge driving circuit so that motor MT can operate in different states. Taking the clockwise rotation state as an example, state control circuit 110 issues corresponding control signals S1-S4 to turn on transistors M2 and M4 and turn off transistors M1 and M3, so a driving current IA flows into node A of the motor MT and out from node B of motor MT. In another example of the counterclockwise rotation state, transistors M1 and M3 are turned on and transistors M2 and M4 are off, so a driving current IB flows into node B of motor MT and then out from node A of motor MT. Driving current detection circuit 120 generates an error signal SE to state control circuit 110 according to a driving current passing through motor MT and a reference voltage Vref. State control circuit 110 linearly controls the conductance of transistor M3/M4 according to the error signal SE, so as to maintain the driving current passing through the motor MT at a specific value.
When a user desires to change the operation state of the motor MT to a desired state, for example, changing from the clockwise rotation state to the stationary state, the user should first make motor MT enter a braking state for a period and then make motor MT enter the desired operation state. This operation sequence is necessary to avoid damaging motor MT or transistors M1-M4. FIG. 2 is a diagram showing waveforms of control signals S3-S4 and waveforms of voltages VA and VB respectively at nodes A and B of motor MT in FIG. 1. Before time T1, motor MT is in the clockwise rotation state. At time T1, prior to enter motor MT into a stationary state for example, motor MT must enter a braking state such that state control circuit 110 issues corresponding control signals S1-S4 to turn on transistors M3 and M4 and turn off transistors M1 and M2. At this moment, motor MT does not receive any driving current from an external circuit; motor MT and turned-on transistors M3 and M4 form a short-circuit loop instead, gradually consuming residual energy stored in the motor MT. As can be expected, the residual current passing through the motor MT gradually decreases. After a period of time, motor MT is controlled to change from the braking state to the stationary state.